A) Field of Invention
The present invention relates to placing radioactive seeds at a cancerous site, and more particularly to a computer-program-implemented technique for determining optimal locations for placing radioactive seeds at a cancerous site.
B) Description of Related Art
At the present time there are many modalities of treatment for cancer. The most basic treatment is surgery. With this treatment modality, the cancerous site is simply excised by operative intervention. This treatment modality is ineffective when the cancerous site is not confined to a specific location, such as in the form of a tumor. Moreover, this treatment modality entails all of the risks and hazards associated with surgical procedures.
Another currently available treatment modality is external-beam radiation therapy. This treatment entails directing radiation beams from an external source to a cancerous site as an attempt to selectively kill the cancerous cells. There are also certain disadvantages of external-beam radiation therapy. One disadvantage is that this treatment kills healthy cells as well as cancerous cells. Also, with external-beam radiation therapy, the patient is required to visit a treatment center five days a week for a six week period.
Both the surgical and radiation treatment modalities are effective if performed at the early stages of the disease. When the disease has metastasized and is in its later stages, the treatment modality with the most promise is chemotherapy. With chemotherapy, agents are injected into the patient which can reach the distant metastatic sites. The agents are essentially toxins which are considered to be more toxic to the cancerous cells than to the normal cells.
Many physicians have also tried hormonal therapy as an adjunct to the surgical and radiation treatment modalities. For example, it is known that prostate cancer grows more rapidly in testosterone-rich environments. As such tumor shrinkage can be achieved by administering testosterone-suppressing hormones. However, hormonal therapy also has certain drawbacks. In particular, although many cancerous cells may die in a testosterone-deprived environment, some tumor cells may continue to thrive. Also, when hormonal therapy is used as a treatment for prostate cancer, a loss of sexual interest may result.
Cryosurgery has also been used for the treatment of cancer. This treatment involves freezing the cancerous tumor. When treating prostate cancer with this technique, needles are inserted into the prostate under the guidance of transrectal ultrasound. Liquid nitrogen is then inserted into the needles and the prostate is cooled to -180 C. Unfortunately, test results concerning cryosurgery reveal that a 30-50% positive biopsy-rate generally occurs six months after surgery.
The treatment modality which is the focus of the current invention is brachytherapy. With brachytherapy, radioactive seeds are placed inside a patient at the cancerous site to provide a radiation therapy. The radiation treatment provided by brachytherapy is largely confined to the cancerous site. Thus, the likelihood of damaging healthy adjacent tissue is minimized.
During the decades before modern imaging techniques became available, treatment with brachytherapy required implanting radioactive seeds by performing an operative procedure on the patient. Modern brachytherapy is, however, often based on less invasive approach. For example, modern brachytherapy treatment for prostate cancer is typically based on ultrasound imaging by using a technique generally known as transrectal ultrasound (TRUS) brachytherapy.
Referring to FIG. 1, conventional TRUS brachytherapy techniques typically require the use of a rectal ultrasound transducer 1, a radiation therapy ultrasound unit 9, and a treatment planning computer 12. As shown in FIG. 2, the transducer 1 includes a probe 3 which is inserted into the rectum and a template 5. The template 5, as shown in FIG. 3, includes a number of holes 8 into which needles 7 are placed. The holes 8 are arranged in a matrix which is defined by rows 0 through 12 and columns A through M. The needles 7, which are hollow, will ultimately be used for placing the radioactive seeds into the prostate.
The transducer 1 captures a series of parallel cross-sectional images of the prostate. Each cross-sectional image captured by the transducer is generally spaced at an interval of 5 mm from the preceding image. For most patients, six to nine images are required to scan the entire prostate. The radiation therapy ultrasound unit 9 contains a screen 11 which shows each of the cross-sectional images of the prostate captured by the transducer 1. A hard copy of each image can be made for treatment planning purposes.
FIG. 4 shows a detailed version of one of the images displayed on the screen 11. The dots shown on the image correspond to the holes 8 in the template 5 where the needles 7 to be used for implanting the radioactive seeds can be inserted. Similar to the template 5, the dots reflect a matrix which is defined by rows 0 through 12 and columns A through M. The vertical and horizontal spacing of the dots is 5 mm.
The radiation therapy ultrasound unit 9 calculates the volume of the prostate based on the measurements contained on each of the cross-sectional images captured by the transducer 1. After this volume is calculated, a physicist then determines a given number, location, and activity of radioactive seeds needed to effectively treat the prostate. This determination is based solely on intuition and experience. The determined number, location and activity of radioactive seeds are then loaded into a treatment planning computer 12. The computer 12 then generates isodose contours based on the number, location, and activity of radioactive seeds for each of the cross-sectional ultrasound images.
FIG. 5 shows a typical set of isodose contours which were generated by the computer 12 and superimposed over the ultrasound image shown in FIG. 4. The outer contour line 13 encloses an area of the prostate which will receive at least 50% of the prescribed radiation dose. The middle contour line 15 encloses an area of the prostate which will receive at least 100% of the prescribed radiation dose. And, the inner multiple contour lines 17 enclose areas of the prostate which will receive at least 150% of the prescribed radiation dose.
The physicist then compares the shape of the isodose contours in FIG. 5 with the prostate contour for each cross-sectional image. The objective of the treatment is to have the isodose contour 15 conform to the shape of the prostate to the highest degree possible. That is, the radiation produced by the seeds should have a minimal encroachment on areas outside the prostate. Also, the physicist must review those portions of the prostate which will receive more than 150% of the prescribed radiation dose to ensure that the patient is not overdosed.
Typically, the physicist has to refine the original prescription several times to generate an acceptable treatment plan. In most instances, it takes numerous iterations and a substantial amount of time to develop an acceptable treatment plan.
Once an acceptable treatment plan is developed, an appropriate number of radioactive seed are placed in needles 7 which are positioned in the appropriate holes within the template. The seeds are then inserted into the patient's prostate and the procedure is complete.
While conventional TRUS brachytherapy has proven to be effective in the treatment of prostate cancer, serious problems persist with this treatment modality. Specifically, conventional TRUS brachytherapy generally requires scanning the patient for ultrasound images on two separate occasions--on a first occasion for obtaining a pretreatment plan and, on a second occasion during which the radioactive seeds are actually implanted.
Given that ultrasound imaging is dependent on the exact position of the transducer 1, the images that a clinician acquires during the pretreatment procedure may differ dramatically from the images acquired during actual implantation procedure. Thus, the treatment plan which is designed to conform to the prostate boundaries formed on the pretreatment images may not conform very well to the images acquired at the time of implantation. This inability to reproduce images during the implantation procedure which are substantially the same as the images formed during the pretreatment procedure results in gross inefficiencies and sub-optimal treatment.
Another difficulty with conventional TRUS brachytherapy for treating prostate cancer is the inability to implement the pretreatment plan. Specifically, ultrasound images obtained for conventional pretreatment plans do not indicate where boney structures are located in relationship to the needle entry points. As a result, in many instances, a needle entry position defined by the pretreatment plan is blocked by a boney structure.
Also, even if TRUS brachytherapy for treating prostate cancer is performed with a conventional treatment planning system by conducting only a single session at the time of implantation and without conducting a pretreatment session, there are still certain drawbacks. In particular, after the prostate volume is measured by the system described above, a prescribed number and location of the radioactive seeds still needs to be determined.
With conventional treatment techniques, this prescription is based on intuition and experience and requires many iterations and thus a large amount of time to develop. Moreover, in many instances, the final prescription may not result in an optimal placement of the seeds. When this occurs, the prostate does not receive the appropriate radiation dose.
In view of these problems, there currently exits a need for a device or method that would allow a clinician to conduct TRUS brachytherapy for prostate cancer only at the time of implantation and in such a manner that the number and location of radioactive seeds will be quickly and precisely determined to optimize the radiation therapy.